Polyvinyl alcohol (PVA), a water-soluble synthetic resin, has been used mainly as a raw material for synthetic fibers. Recently, it is used in various fields, for example for production of film materials, emulsifying dispersants, adhesives and binder resins, because of its favorable properties. The PVA resins are generally produced by polymerization of a vinyl ester and saponification of the resulting polyvinyl ester in an organic solvent in the presence of a catalyst.
Normally in methods of producing a PVA resin, a monomer-removing step of removing unreacted monomers from the polymerization solution is carried out between polymerization and saponification steps. Examples of the method include feeding the polymerization solution obtained in polymerization step into a monomer-removing column having trays in multiple stages, and bringing methanol into contact with the polymerization solution, as methanol vapor is injected into the column from the column bottom (see Patent Documents 1 and 2).
On the other hand, perforated-plate columns having multiple compartment walls formed therein have been proposed as the monomer-removing column for use during production of polyvinyl chloride resins (see, for example, Patent Document 3). In the monomer-removing column described in Patent Document 3, a channel for treatment of the polymerization solution is formed with compartment walls, wherein the contact time between the polymerization solution and steam is regulated.